Remove uses of NEEDS_DISPATCH_RETAIN_RELEASE. Use OS_OBJECT_USE_OBJC.

According to os/object.h the macro OS_OBJECT_USE_OBJC can be used to determine
if the compilation target supports libSystem objects be used as Obj-C types.
Merge pull request #139 from drodriguez/bye-bye-needs-dispatch-retain-release

Remove uses of NEEDS_DISPATCH_RETAIN_RELEASE. Use OS_OBJECT_USE_OBJC.